Thermal Bridging Research: Rainscreens

One of our many in-house research efforts is a large, collaborative project. We investigated the Thermal Performance of Façades through the AIA Upjohn Research Initiative.

Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. These thermal bridges are most often caused by structural elements that are used to transfer loads from the building envelope back to the building superstructure. Though design professionals generally understand that thermal bridging is a concern, few can quantify the extent of its impact on building performance.

Small changes in designs can still lead to dramatic improvements in performance. With careful detailing and attention to the issues of thermal bridging, the design and construction industry can improve the performance of our building envelopes.

Today we’re sharing our findings regarding rainscreens.

Rainscreens have become increasingly popular for commercial façades in the past few decades due to their ability to control air and moisture movement. Because the cladding is held off the wall structure to form a drainage cavity while accommodating insulation and a robust air and vapor barrier, these systems require a secondary structural system of rails, Z-girts and/or clips to support the cladding. Typically made of highly conductive metals, these structural members penetrate through the insulation and cause significant thermal bridges. While insulation between steel studs has long been acknowledged in the industry to cause thermal bridging, these rainscreen supports have a similar thermal impact that was widely overlooked until recently.

In our thermal images of rainscreen façades, we observed a decrease in thermal performance that ranged from 20% to 60% less than the design intended performance, with a typical decrease of 45-55%. All of the systems we selected for study had between two to three inches of insulation. We looked at a rainscreen with horizontal Z-girts, vertical Z-girts and a clip-based system. Not surprisingly, the continuous Z-girts, whether horizontal or vertical, performed similarly. In both orientations, Z-girts demonstrated an R.7.7 reduction in the assembly’s R-value, or an approximate 45-55% reduction in performance depending on the insulation thickness.

The façade with the clip system for the rainscreen performed much better than those with continuous Z-girts. Because of the intermittent nature of the clips, these systems performed well both in thermal images and in the computer modeling. The clip support system had half of the heat flow of the Z-girts, or 25% of the design intent. While the intermittent nature of the support system certainly improved the performance, we investigated ways to further improve the performance of rainscreen support systems.

Examples of thermally broken rainscreen supports

A number of thermally broken Z-girt and rainscreen support systems currently exist on the market. As part of the research project, the team explored three of the available thermally-broken options. The first removed the support through the insulation with horizontal and vertical tube supports on the exterior and allowed only the stainless steel bolts to penetrate the insulation. The second system investigated a fiber glass clip system. This has the benefit of being intermittent, similar to the previous clip, but also uses a material that is more than 200 times less conductive than steel. The third system investigated was a discontinuous steel bracket with isolator pads on both the warm and cold side of the insulation, in order to minimize heat flow through the brackets. All three of the tested systems performed well. In general, the R-value of the assemblies was only reduced by 10-15% due to thermal bridging through their support systems, and so they achieved a minimum of R-20 with four inches of insulation.

Related:
Thermal Performance of Facades: Final Report
Thermal Bridging Research: Curtain Walls
Thermal Bridging Research: Investigating Insulation Thickness for Renovations
Thermal Bridging Research: Masonry Veneer Walls
Thermal Bridging Research: Window Transitions
Thermal Bridging Research: Foundation to Wall Transitions